Synthetic lubricating oil composition



SYNTHETIC LUBRICATING 01L COMPOSITION David W. Young and Robert L. May,Homewood, 111.,

assignors to Sinclair Refining Company, New York,

N.Y., a corporation of Maine No Drawing. Application May 18, 1956 SerialNo. 585,609 4 Claims. (Cl. 252-56) This invention relates to novelcompositions of matter. More particularly, the invention is concernedwith reaction products prepared from dibasic acids or their esters andglycols. The reaction is continued until the products have desiredviscosities which make them useful as lubricants and particularly assynthetic lubricant oil additives or blending components. The newproducts are particularly useful as viscosity improvers and loadcarrying agents for synthetic lubricants and as intermediates forpreparing materials having these properties.

The products of this invention are polyesters and can be prepared bydirect esterification of dibasic acids with glycols and by an esterinterchange reaction between an ester of the dibasic acid and theglycol. These reactions are continued until the product has a kinematicviscosity from about 15 to 200 centistokes at 210 F. and preferablyabout 40 to 125 centistokes at 210 F. The products themselves can beemployed as lubricants but in particular they are useful as additives orblending components in synthetic lubricant compositions. Preferably, thenew reaction products are blended with diester synthetic lubricantsprepared from dibasic acids and low molecular Weight alcohols. Thus theviscosity of our product might be lowered by the addition of the diesterlubricant or our product can be employed to thicken the diester basestock. In the latter blends the reaction product of the presentinvention will increase the load carrying capacity of the base diesteroil and the thickener would generally not be more than about 50 weightpercent of the blend, preferably about 20 to 50 weight percent. Usuallythe amount of the reaction product employed in any blend would be atleast about percent and advantageously the final lubricating oilcomposition would have a maximum viscosity at -40 F. of about 13,000centistokes and a minimum viscosity of about 7.5 centistokes at 210 F.

The dibasic acid employed in preparing the reaction products of thisinvention will in general contain from about 2 to 28 carbon atoms.Suitable acids are described in US. Patent No. 2,575,195 and include thealiphatic dibasic acids of branched and straight chain structures whichare saturated or unsaturated. The preferred acids are the saturatedaliphatic dibasic acids containing not more than about 12 carbon atoms.Such acids include succinic, adipic, suberic, azelaic, sebacic, andisosebacic acids which are mixtures of a-ethyl suberic acid,u,u'-diethyl adipic acid and sebacic acid. Others of the dibasic acidsinclude brassic, brassylic, pentadecanedicarboxylic acid,tetra'eicosanedicarboxylic acid, C to C alkenylsuccinic acids,diglycolic acid, and thiodiglycolic acid.

When our reaction products are made by an ester interchange reaction, weemploy the esters of the above described dibasic acids. The esters usedare those of the acids and the lower aliphatic alcohols so that thealcohol produced in the reaction will be volatilized under the reactionconditions. Generally, the alcohol group of the ester contains up toabout 5 carbon atoms and preferably from about 1 to 3 carbon atoms.Suitable esters include methyl sebacate, ethyl adipate, propyl azelate,isopropyl succinate, butyl sebacate, pentyl suc- United States Patent2,929,786 Patented Mar. 22,

2 cinate, methyl brassylate, and similar esters of the other dibasicacids.

In preparing the reaction products of the present 1nvention 1 mole ofacid or its ester is reacted with about 0.7 to 1.5 moles of glycol, andpreferably this ratio is about 1 to 1. The useful glycols include thealiphatic monoglycols of 3 to 12 carbon atoms, preferably 3 to ,6 carbonatoms; and the polyglycols thereof having from about 1 to 50 etheroxygen atoms. Advantageously, the polyglycols contain from about 1 to 6ether oxygen atoms. The preferred polyglycols are the polypropyleneglycols and particularly useful ones of these have molecular weightsfrom about to 450. Others of the glycols are proplyene glycol, butyleneglycol, polybutylene glycols, decamethylene glycols and octyleneglycols, Preferably, the glycol hydroxy radicals are primary or terminalbut they can be otherwise placed in the molecule. Also, the glycols maycontain thioether linkages and can be straight or branched chain, forinstance, as in 2-ethyl-1,3-hexanediol, 2-methyl-l,3-pentanediol,2-propyl-1,3-heptanediol and2-butyl-1,3-butanediol. Minor amounts ofother glycols or other materials can be present as long as the desiredproperties of the product are not unduly deleteriously affected, e.g'.its solubility in the synthetic oil base should not be destroyed.

When the esterification reaction is conducted between a dibasic acid andthe glycol it is continued with concomitant boiling-off of water fromthe reaction mixture until the product has the desired viscosity. Thetemperature of this reaction is usually at least about 300 F. and shouldnot be so high as to decompose the wanted product. If desired, thereaction can be conducted in the presence of a solvent, for instance anaromatic hydro'carbon such as xylene, and to provide a betterreactionrate we prefer to employ an acid esterification catalyst. Many of thesecatalysts are known and include, for instance hydrochloric acid,sulfuric acid, aliphatic and aromatic sulfonic acids, phosphoric acid,perchloric acid, hydrobromic acid, hydrofluoric acid anddihydroxyfluoboric acid. Other catalysts are thionyl chloride, borontrifluoride, silicon tetrafluoride, the chlorides of magnesium,aluminum, iron, zinc, copper and tin and salts of mercury, silver,cobalt, nickel and cerium. In the preferred reaction, when employing thedibasic acid, we use about 0.1 to 0.5 weight percent of paratoluenesulfonic acid catalyst, a xylene solvent and a temperature of about 345to 390 F. while boiling-01f water by refluxing.

When employing the esterification or ester interchange alcoholysisreaction between the dibasic acid ester and the glycol, we prefer not touse a solvent and the temperature is generally above 350 F., but not sohigh as to decompose the wanted product. Advantageously, the temperatureis in the range of about 435 to 480 F. Many ester exchange catalysts areknown and include for instance zinc stearate, aluminum stearate,dibutyltin oxide, titanium tetraesters of lower aliphatic alcohols,sodium acid sulfate, sulfuric, hydrochloric and sulfonic acids, aluminumalkoxides, sodium methyl carbonate. Also, these catalysts areexemplified by the alkali metal and alkaline earth metal alkoxides,hydroxides and carbonates.

Our new reaction products can be blended with various synthetic oils oflubricating viscosity, for instance ranging from the light to heavy oilshaving viscosities of about 35 SUS at 210 F. to 250 SUS at 210 F., andpreferably about 35 to 150 SUS at 210 F. Included among the syntheticlubricants are, for example, the polyalkylene glycols prepared fromalcohols and alkylene oxides. A number of these oils are available asUcons prepared from aliphatic alcohols and propylene oxide 3 apd havingmolecular weights of about 400 to about 3000. Related lubricants are thehigher molecular weight alkylene oxides, polycarbonates, acryloidpolymers, formalaro y m l po ys yeidvl thers a d ot er eth r;-

Widely employed synthetic lubricating oils arethe ester types, forinstance the monoand diesters. The esters are; made from alcohols andeither monoor polycarboxylic acids. Arnongthe dicanboxylic acidsemployed arethose of up to about 12 carbon atoms, particularly theticbase oils consist essentially of carbon, hydrogen andogygen, iQe., theessentialnuclear chemical structure formed by these elements alope.However, these oils may be substituted with other elements such, ashalogens, e-s b er n and flu n Among other specific synthetic oilsfalling within the above classes are ethyl palmitate, ethyl stearate,di-(Z- ethylhexyl) 'sebacate, ethylene glycol di-laurate, di-(2-ethylhexyl) phthalate, l-naphthyl cetyl ether, di-cety1 ether,polypropylene glycol (and monoand diethers) (molecular weight 600),di-(l,3-dimethyl butyl) adipate, di (2-ethyl butyl) adipate, di-(l-ethylpropyl) adipate, diethyl oxalate, glycerol tri-n-octoate, di-cyclohexyladipate, di(undecyl) sebacate, tetraethylene glycol-di-(2- ethylenehexoate), di-cellosolve phthalate, butyl phthallyl bptyl glycolate,di-n-he xyl fumarate polymer, dibenzyl sebacate, diethylene glycolbis(2-n-butoxy ethyl carbonate) and oxo process alcohols such asiso-octyl, isodecyl and. isotridecyl alcohols made from branched chainpropylene polymers. r

' When desired our reaction product polyesters themselves or theirblends including another lubricating oil can coutain additives orcomponents to impart wanted characteristics to the compositions. Forinstance, antioxidants, extr eme pressure or load carrying agents,antifoaming agents, corrosion inhibitors, etc., can be added.

The following specific examples will serve toillustrate the products ofthe present invention but they are not to be considered limiting.

EXAMPLE I 1,212 grams of sebacic acid, 900 grams of polypropylene glycol150 average molecular weight,'700 grams of xylene, and 11 grams ofp-toluenesulfonic acid were placed in a 5 liter three-necked flaskfitted with a thermometer, heating mantle, glass stirrer, refluxcondenser, and water-trap. This mass was stirred and. heated to reflux,230 F. Water came over at once. Refluxing was continued for 7 hours as210 cc. of water was collected. The final reflux temperature was 315 F.The mass which was our new reaction product dissolved in xylene solventwas then cooled to room temperature. The product was further modified byfirst adding 211 grams of 2-ethyl hexanol. The mixture was stirred andheated to reflux, 315 F. Water came off at once again. Refluxing wascontinued for 5 hours as 9 cc. of water was collected. The final refluxtemperature was 325 F. The reaction was stopped and the total watercollected was 219 cc. The resulting prodnot was then topped to 400 F. at2-5 mm. Hg to remove xylene. The topped ester and 10% of propylene oxidewere placed in a 1 liter stirred autoclave, and stirred and heated to400 F. It was held at this temperature for- 4 hours. The maximumgaugepressure was 110 pounds. The mass was then taken from. t b b andstirred and heated at 356 F. in an open beaker for 15 minutes to flashoff the excess" propylene oxide and other volatile matter. The liquidproduct was then filtered and had an acid number, ASTM-D 974, of 0.2 anda K.V. at 210 F. of 72.12 cs.

EXAMPLE II 607 grams of sebacic acid, 1,275 grams of polypropyleneglycol 425 average molecular wegiht, 700 ml. of xylene, and 3.76 gramsof p-toluenesulfonic acid were placed in the same equipment as used inExample I. This mixture washeated and stirred at reflux (320-365 F.) for24 hours. Water collection was essentially nil after this time and ournew product was in solution in the xylene. The product was furthermodified by adding 188 grams of 2-ethyl hexanol and the mass was stirredand heated at 356 F. for 12 hours during which time a little more watercame ed. The product was topped to 400 F. at 2 to 5 mm. Hg to removexylene. The resulting liquid polyester was then filtered and the acidnumber, ASTM-D- 974, was 6.47. The product had an actual pour pointof.-44 F. and viscosities of 344.3 cs., K.V. at .100 F. and 41.4 cs.,K.V. at 210 F.

EXAMPLE In 226 grams of azelaic acid, 2,430 grams of polypropyleneglycol 2025 average molecular weight, 900 grams of xylene, and 14 gramsof p-toluenesulfonic acid were placed in the same equipment as used inExample I. This mixture was stirred and heated to reflux, 340 F. Watercame over at once. Refiuxing was continued for 6 hours as 47 cc. ofWater was collected. The final reflux temperature was 345 F. The masswhich was our new reaction product dissolved in xylene was then cooledto room temperature. The product was further modified by first adding266 grams (10%) of Z-ethyl hexanol. The mixture was stirred and heatedto reflux, 345 F. Water came 0d at once again. Refluxing was continuedfor 7 hours as 6 cc. of water was. collected. The final refluxtemperature was 345 F. The reaction was stopped and the total Watercollected was 53 cc. The resulting product was then topped to 400 F. at2-5 mm. Hg to remove xylene. The topped ester and 10% of propylene oxidewere placed in a 1 liter stirred autoclave, and stirred and heated to400 F. It was held at this temperature for 4 hours. The maximum guagepressure was pounds. The mass was then taken from the. bomb and stirredand heated at 356 F. in an open beaker. for 15 minutes to flash off theexcess propylene oxide and other volatile matter. The liquid product wasthen filtered and had a K.V. at 210 F. of 36.80 cs.

EXAMPLE IV 565 g. of azelaic acid, 1,275 g. of polypropylene glycol 425average molecular weight, 500 g. of xylene, and 1 g. ofp-toluenesulfonic acid were placed in the same equipment as used inExample I. This mixture was heated and stirred at 356 F. (reflux) for 24hours. Water collection Was essentially nil after this time. The productwas then topped to 400 F. at 2-5 mm. Hg to remove xylene. The liquidmaterial was our new reac tion product and it had an acid number of13.88 and a viscosity at 210 F. of 23.00 cs. This material was rechargedto the equipment and 500 ml. of xylene was added. 10% (on the weight ofpolyester) of 2-ethyl hexanol was also added. This mass was then stirredand heated at 356 F. for 12 hours and more water was collected.Theproduct was topped to 400 F. at 2-5 mm. Hg to remove xylene and theliquid was filtered.

Inspections on our new reaction product were:

K.V. at 100 F cs 182.4 K.V.'at 210 F .cs 23.45 Actual pour, F .64

To provide a lubricating oil blend, 350 g. of the reaction product isblended with 650 g. otPlexol 201, a di-2- EXAMPLE V 321 g. of adipicacid, 2,255 g. of polypropylene glycol H 1025 average molecular weight,700 g. of xylene, and 13 g. of p-toluenesulfonic acid were placed in thesame equipment as used in Example I. The mixture was stirred and heatedto reflux, 290 F. Water came off at once. Refiuxing was continued for 7hours as 83 cc. ofwaterwas removed. The final reflux temperature was 324F. The mass which was our new reaction product dissolved in xylene wascooled to room temperature. The prod uct was further modified -by firstadding 258 g. (10%) of 2-ethyl 'hexanol. The mixture was then stirredand heated to reflux, 326 F. Water came over again at once. Refiuxingwas continued for ,7 hours as 6 cc. of water was collected. Thefinalreflux temperature was 339 F. The reaction was stopped and thetotal water collected was'89 cc. The resulting product was topped to 400F. at 2-5 mm. Hg to remove xylene. 1,354 g. of the topped ester and 135g. of propylene oxide were charged to a 2 liter stirred autoclave. The.mix-.

ture was heated to 400 F. and held there for 3 hours. 3 The maximumguage pressure. was 50 lbs. The product EXAMPLE VI 454 g. of1,3-pentanediol, 620 g. of sebacic acid, 500 g. of xylene, and 3 g. ofp-toluenesulfonic acid were charged in the same equipment as used inExample I; The mass was stirred and heated to 320 to 356 F. and.

held there for 24 hours. Water boil-off was approximately zero afterthis time. The product was filtered and topped to 400 F. at 5 mm. Hg toremove xylene.

' 6 K.V. at 100 F M 148.85 K.V. at 210 F- 19.00 Actual pour, F- 62 AcidN0. ASTM-D 974 0.143

A lubricating oil blend can be provided by adding 200 g. of this productto 800 g. of 'Plexol 201.

' EXAMPLE VII- 1,000 g. of redistilled dimethyl sebacate, 1,850 g. ofpolypropylene glycol 425 average molecular weight, and 1.4 g. of Paranox441 (2,6-di-tertiary-butyl-p-cresol oxi-- dation inhibitor) were chargedto a 5 liter flask fitted with a glass stirrer, heating mantle,thermometer, reflux condenser and trap. The system was flushed withnitrogen and the reaction was carried out under a nitrogen atmosphere.The reagents were heated to "342 F. and

, 4 g. of tetraisopropyl titanate were added rapidly below Laboratoryinspections on our new reaction product were:

K.V. at 100 F- cs 150.3 K.V. at 210 F cs 19.12 Actual pour, F- 62 AcidNo. ASTM-D 974 28.27

To provide a lubricating oil blend, 250 g. of the reaction product isblended with 750 g. of di-(isooctyl) azelatc. The product was thencharged to a 1 liter stirred autoclave.- 18% by weight of propyleneoxide was added. The mass was stirred and heated at 400 F. for 6 hourswith a maximum guage pressure of 110 lbs. The product was removed,filtered and analyzed:

the surface of the reactants. The mass was then heated at above 355 F.and at atmospheric pressure for 3 hours. The maximum temperature was 453F. The product was thenheated at. 6.5 mm. Hg at temperatures up to 388F. until no more alcohol make was observed. The liquid product wasfiltered and had a K.V. at 210 F. .of

EXAMPLE VIII 0 of ib y s b cate. 3. .,sp p pylene glycol 425 averagemolecular weight, and 23.6 g. of tetraisopropyl titanate were charged tothe same equipment as in Example VII. The mass was heated to 235 to 245C. and kept there for 2% hours. Oxygen-free nitrogen was added at a 2cu.-ft./hr. rate to help remove the alcohol of reaction. The sample wasthen fil d and test data were: 1

K.V. at F M 211.4 K.V. at 210 F N 30.62 Actual pour, F --42 Acid No.ASTM-D974 3.55

K.V. at 100 F cs 207.4 K.V. at 210 F 29.75 Actual pour, F' 42 Acid No.ASTM-D974 0.152

A lubricating oil blend can be provided by adding 450 g. of this productto 550 g. of di-(isooctyl)azelate.

The pertinent data for additional polyester preparations are listed inTable I. The experimental procedure was essentially that of Example VII.

Table I ESTER PREPARATION BY INTEROHANGE REACTION Examples IX X XI XIIXIII Reagents:

Ester Dlmethyl Dimethyl Dimethyl Dimethyl Dimethyl sebacate. sebacate.sebacate. sebacate. sebacate Poly Propylene Glycol, average M.W. 425 42525 025.

M01. Ratio, Ester/Glycol 1.0. 1.0.- 1.0.. 1.0.. 1.0

Paranox 441, wt. per n Catalyst wt. percent .1--- .72.. .18.. .19.. .19.

Catalyst wt. per n Maximum Reaction Temp, C 245 240 266 250 254. Hoursabove 180 0 5 4.5.- 3.5.- 2.2- 3.1. Finishing Vacuum, mm. H2 6 100 '-5 55. Finishing Temp, 0.. 204 200 210 ,202 200, Product:

K.V. at 100 F. (05.) 358.2- 6333.-----" 876.5 879 293.7.

K.V. at 210 F. (08.) 47.32. 73.72 9l.12..-. 109.86.-.---- 42.95.

Acid Number ASTM-D974 1.05-- .73.. .28.. .08.- .21.

Actual Pour, F- 48 40 35 44 45.

Table. l- -Contmucd Examples XIV" XV XVI 1 XVII XVIII Reagents:

Ester Dimethyl Di-n-butyl Dimethyl Dlmethyl Dime'thyl sebacate.sebacate. sebacato. sebacate. sebacate. Polylropylene Glycol, averagervnw. 425; 42R 425 g R 425. Mo}. Ratio, Ester/Glycol 1.0-- 1.0--. 0.8.-1.25-- 1.0. Paranox441, wtwper .05 .05.- 06- Catalyst wt. permmt 0.820.Catalyst wt.per Maximum Reaction Temp., 0.--. 230, Hours Above 180 O IFinishing Vacuum mm. Hg 5, Finishing Temp C 2011' Product: 1

KN. at 100 F. (cs) 242.50.. 121.7. K.V. at. 210 F. (cS.) 38.59--.--18.4. Acid Number A8TM-D974 .74 .28 Actual Pour, 3..

1 Tetraisopropyl tltanate. 1 Dibutyl tin oxide.

Typical lubricating oil blends containing the products of a number ofthe above'examplcs are described in Table 11.

with a material selected from the groupconsisting of monoglycols of 3 to'6 carbon atoms and. polyglycols thereof having up to about 6 etheroxygenatomsj,

Table II SYNTHETIC OIL BLENDS Composition, .Wt. Percent x 201 67.9 73.7272.8 72.8 53.9 73.72 40. 75 64 55.3 70.5 73.5 Phenothiazine 1.1-- 0. 50. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 5 0. 6 Trlcresylphosphate 112. 5 2. 5 2. 5 2. 5 5. 0 2. 5 2. 5 2. 5 D.C 200-6 00 d 0.001 0.001 0.0010.001 0.001 0.001 0. 001 0.001 0. 001 0.001 0.001 Ester ThickeneL- 20. 123. 28 24. 2' 24.2 40. 6 23. 28 49. 75 33 41. 7 23 26 T ts of Example IXX XV VII II I IV 111'. V XVI XVII K.V. F 10,005 9, 044 14.204 10, 89610,000 8,211 21,305 11,272 10, 742 8,007 7, 515

K.V. @100 F 35. 75 3 1. 03 51. 47 40. 34 52. 4 32. 88 43. 19 34 40. 8133. 6 1 33.16

KN 210 F 7. 694 7. 596 11. 03 8,808 10. 08 7. 126 8. 469 8. 096 8. 471.7. 490 7. 837

Pour, below S0 below 80 below below below below below below below belowbelow Aold Number, ASTM-D974. 0. 28 O. 11 0.20 0 03 2. 72 O 12 0 0. 060. 01 0. 04 0. 13 Ryder Gear Test pounds 4, 000-4, 170 4, 020-4, 040

I Di-2-ethylhexyl seba'cate oil has a KN". 100 F; 01 12.3 05.; V1. of154; acid number of 0.12 and pour below 80 F.

b Anti-oxidant. Load-carrying agent.

We claim:

1. A synthetic lubricating oil composition having a maximum viscosity at-.40 F. of about 13,000 centistoks and a minimum viscosity of about 7.5centistokes at- 210 F. and consisting essentially ofran oily diesterbase having aviscosity of about 35 to 150 SUS at 210 F, said diestcrbeing of an aliphatic dibasic acid of up to about 12 carbon atoms and analiphatic alcohol of up to about 12 carbon atoms, and about 20 to 50Weight percent of the polyester produced by reacting in the mole ratioof about 1:1 sebacic acid and polypropylene glycol having a molecularweight of about 150 to 450, said polyester having a kinematic viscosityof about 40 to 125 V centistokes at 210 F.

' 2. A synthetic lubricating oil composition having a maximum viscosityat --40 F. of about 13,000 centicent of the. polyester produced byreacting in the molar ratio of about 1:1 amaterial selected from the'group-- consisting of aliphatic dibas iclacids of up to about 1 2carbon atoms and the lower aliphatic esters thereof containing up toabout 3 carbon atoms in the ester group ming agent.

polyester having a kinematic viscosity of about 40 to centistokes at 210F.

3. The composition of claim 2 in which the polyester is prepared fromsebacic acid and polypropylene glycol having a molecularweight of aboutto 450.

4. A synthetic lubricating oil composition .having a maximum viscosityat -40 F. of about 13,000 centistokes and a minimum viscosity of about7.5 centistokcs at 210 F. and consisting essentially of di-2-cthylhc1iy1scbacate base oil and about 20 to 50 weight percentof the polyesterproduced by reacting in the mole ratio of about 1:1 sebacic acid andpolypropylene glycol having a molecular weight of about 150 to 450, saidpolyester havinga kinematic viscosity of about 40 to 125 centistokcs at210 F.'

References Cited in the tile of this patent UNITED STATES PATENTS2,499,984 Beavers et a1. Q Mar. 7, 1950 2,562,878 Blair Aug. 7, 19512,628,974 Sanderson Feb. 7, 1953 2,744,025 Albus et al May 1, 19562,785,194 Hoare Mar. 12, 1957 f FOREIGN PATENTS 111,211 Great BritainJune 30, 1954

2. A SYNTHETIC LUBRICATING OIL COMPOSITION HAVING A MAXIMUM VISCOSITY AT-40*F. OF ABOUT 13,000 CENTISTOKES AND A MINIMUM VISCOSITY OF ABOUT 7.5CENTISTOKES AT 210*F. AND CONSISTING ESSENTIALLY OF AN OIL DIESTER BASEHAVING A VISCOSITY OF ABOUT 35 TO 250 SUS AT 210*F., SAID DIESTER BEINGOF AN ALIPHATIC DIBASIC ACID OF UP TO ABOUT 12 CARBON ATOMS AND ANALIPHATIC ALCOHOL OF UP TO ABOUT 12 CARBON ATOMS, AND ABOUT 20 TO 50WEIGHT PERCENT OF THE POLYESTER PRODUCED BY REACTING IN THE MOLAR RATIOOF ABOUT 1:1 A MATERIAL SELECTED FROM THE GROUP CONSISTING OF ALIPHATICDIBASIC ACIDS OF UP TO ABOUT 12 CARBON ATOMS AND THE LOWER ALIPHATICESTERS THEREOF CONPOLYESTER UP TO ABOUT 3 CARBON ATOMS IN THE ESTERGROUP POLYESTER HAVING A KINEMATIC VISCOSITY OF ABOUT 40 TO 125CENTISTOKES AT 210*F.